Offshore construction for petroleum production is in a growing boom; much of the work includes laying submarine pipelines. Thousands of miles of submarine pipelines have been laid in the past few years, presenting many problems which remain to be solved. One of the most serious is repair. Pipelines deteriorate and they are damaged, as by fishing trawls, ship anchors, scouring away of support, transport by currents, and by submarine landslides. The direct cost of repair is high. It is very often done by use of vessels and associated equipment costing more than $100,000 rental per day, with the total effort often requiring weeks, resulting in multi-million dollar direct costs. The indirect costs in lost product, lost time, and damage to environment are usually much greater.
There follows a brief description and analysis of six prior art methods now in use to repair submarine pipelines:
1. Picking up the line: When the sea is fairly calm and not too deep, and in the case of puncture or leaking of a pipeline, a derrick barge will go the site and by use of several hoisting means at once, lift the damaged part of the pipeline clear of the water, with the remainder draped away from the barge. The damaged part is then patched by wellknown means, such as welding, and the repaired line is then lowered back to the marine floor.
The deeper the water the greater the likelihood of inducing other damage, as by pulling the line in two or by buckling it, also, in case of buried pipelines, considerable excavation may be required. The above method is most suitable for calm seas, and unburied small lines in shallow water.
2. The "Hard Flange": An old and common method, in the case of somewhat deeper water, and perhaps in the case of complete rupture as by landslide, is to pick up, by derrick barge, two loose ends of the pipeline, perhaps after cutting away of the damaged part, just as the damaged section is picked up in the first method above. The ends are then fitted by welding with flanges, perhaps conventional flanges having bolt holes therearound, or perhaps flanges such as are made by Cameron Iron Works in Houston, Tex. which allow quick connection.
With the flanged end prepared, the line end is laid back on the marine floor. When two ends are so laid, a diver will go down to the bottom, gauge the gap between the ends, and send the results to workmen on the surface who will prepare a segment of pipe with two flanged ends (a "spool piece") to suit the guaging. The spool piece is then lowered to the diver, who must fit it between the two pipeline ends, bolt it or otherwise connect it thereto.
The "Hard Flange" method is a good method where it can be applied. It has the notable advantages of providing a simple strong reliable connection with metal-to-metal seal. Certain difficulties restrict its uses: (a) in case of deep or rough water, there is danger of buckling or otherwise damaging or overstressing the pipe in lifting it to the surface; (b) in case of buried pipelines, considerable excavation is required; or (c) in case of large pipelines, a large derrick barge may be required to lift so much pipe, and large derrick barges are costly, slow and not always available.
3. Adjustable mechanical connectors: HydroTech Systems makes a family of couplings, including one called the "Hydrocouple" which slips over the end of a pipeline and will mechanically connect to and seal onto the line. It allows axial adjustment and angular misalignment of the pipe ends. It will connect underwater to a bare pipe. This makes it very useful for repairs, and, to some extent in some cases overcomes the difficulties (a), (b) and (c) above. The Hydrocouple makes repairs possible without lifting or moving a great length of line. However, it introduces problems of its own: high cost (several hundred thousand dollars per set for large sizes), long delivery, potential leaks, and problems of handling underwater (it is heavy and requires precise positioning movement).
Other manufacturers are preparing to enter this market, but for the present purposes of disclosure they are comparable in value, and in problems introduced, to the Hydrocouple.
4. Welding in a Habitat: About the same time the Hydrocouple was being developed, several companies were developing underwater welding apparatus, including an assembly of huge alignment and handling frames for picking up and aligning the pipeline to be repaired, and a chamber fitted into the alignment frames. The chamber and the pipe are brought into alignment, with the pipeline passing through the chamber. The chamber is sealed and purged of water by air under pressure, providing a pressurized air environment for workmen in the chamber to gain access to the pipes for patching by welding.
These habitat frames work, but they are enormously large, heavy, expensive, and complicated, and repairs made therewith are consequently slow to mobilize and expensive to use, frequently costing several million dollars per repair. The repairs made may be poor, due to bad working conditions for welding and due to the difficulty of hyperbaric welding.
5. One atmosphere welding chambers: Lockheed Petroleum Services has devised one atmosphere chambers for installing connections on pipeline ends, overcoming certain difficulties of the previous method, but with the disadvantage of requiring the abandonment of the chamber whenever it is used. It is a costly method, adapted only to great depths.
6. Wet welding: Several companies have used a method of welding under water. The welds are not as strong as is desired, but will serve in some cases. An underwater weld is tested by using the pipeline. Thus if the weld is weak and ruptures, the repair must be made again.
At the present time all repair work on submerged pipelines is carried out on a cost plus basis, thus the ultimate loser is the consuming public.
In the preceding description of the prior art methods of repair of submerged pipelines, no deprecation is made or intended. Each of these methods has served and will continue to serve the essential task of pipeline repair. Great engineering feats have been performed to maintain the flow of oil and gas by devising techniques and equipment for use in at most hostile and difficult environments.
For better understanding, and to establish vocabulary, a brief review of prior art pipelaying practice follows, applied to a 36 inch diameter pipeline, 5 miles long, laid in water about 250 feet deep, for example: A "pipe lay barge" being a non-self propelled vessel a few hundred feet long having a "stinger" (a long curved adjustable guide truss means cantilevered off of the stern) is loaded with several hundred "joints" (40 foot lengths) of "weight-coated" (covered with about four inches of wire-mesh reinforced concrete all around the middle 38 feet of the joint) pipe.
The barge is then towed to the site of beginning, being a point at the shallow end of the line. A flange is welded onto a joint of pipe, and another joint welded to its other end, and other joints onto it in succession with the resultant beginning of a pipeline being fed or paid out the rear of the barge supported by the stinger. A long line is attached to the end of the pipe and it is hauled off the back of the barge over the stinger (by a tug or winch mounted on a platform) while the barge hauls itself the other way on its anchor cables or by other means such as a tugboat. Tension is maintained by the anchor cables to keep the line from bending over the end of the stinger under its own weight. When the process has continued long enough some length of pipeline will rest on the sea bead and new joints will be welded on and, the pipeline paid out until the intended destination is reached or until some interruption, such as a storm, in either case, the end of the line is dropped to the marine floor, preferably with a flange attached.
Problems of interest here include the problem of maintaining correct tension in the line along with a proper inclination of the stinger so that the line will not be damaged by bending (too little tension) or by pulling (too much tension). If damage should occur, repairs will be required, even on the new line, or a new line must be laid. Also, the problem resulting from having to drop the line, in case of trouble, before a flange can be attached, can be serious. The water may be too deep to pick up the pipe to continue laying, and the end is not prepared for connection.
The aforementioned methods are all well known in the art, and it is with a view to current problems that the present invention is advanced. The present invention offers advantage in cost, reliability, speed, safety, and protection of the environment, requiring only minimal cost and foresight to implement.
It is a particular advantage of the present invention that a means of making reliable metal-to-metal seals underwater is provided.
It is another advantage of the present invention that a method of predetermining the amount of pipe to be required in repairing a pipeline is provided.
It is a particular feature of the present invention that a minimum amount of handling of the pipe during repair is required.
It is another feature of the present invention that pipelines incorporating the invention may be laid using substantially the same equipment, such as barges, tugs, stingers, and the like used in the prior art.